1. Field of the Invention
This invention relates to a high-efficiency electronic circuit for generating and regulating a supply voltage.
More particularly, the invention relates to a circuit as above, comprising a charge-pump voltage multiplier which is associated with an oscillator and has an output connected to a voltage regulator in order to provide said supply voltage on its output.
Particularly but not exclusively, this invention relates to a DC_low_V-DC_high_V voltage converter, as well as to an associated regulating circuit portion which automatically optimizes the converter efficiency according to the applied current load. The description that follows is made with reference to this application field for description simplicity.
2. Description of the Related Art
Nowadays an ever more pressing demand exists in this particular technical field for devices which can be powered at a very low voltage (1 or 2 Volts, for example) from a battery supply in portable apparatus.
This demand clashes with the need to have higher internal voltages (upward of 10 Volts, for example) than the supply voltage available in electronic circuits, for supplying circuit portions which can properly operate only on a higher voltage. Quite often this need is filled by using voltage-multiplying circuits, or charge-pump boosters, which are able to produce a higher voltage than the supply voltage across a charge capacitor
In this context, it is particularly important to have the possibility to regulate and program the supply voltage, on which the boosted voltage is also dependent.
Furthermore, the need of reducing power consumption as much as possible, to extend the battery life, leads to the efficiency optimization of the voltage multiplier, and more generally of all the DC_low_V and DC-high-V conversion circuits. This efficiency is especially restrained by the use of integrated capacitances.
A common approach to generating a voltage higher than the supply voltage is schematically shown in FIG. 1. FIG. 1 is a block diagram of a charge-pump voltage multiplier circuit (x2, x3 or x4, for example) having an oscillator CLOCK associated therewith and an output VCHARGE which is beset with ripple at the scan frequency of the oscillator. A linear regulator connected downstream of the charge pump operates to smooth the signal VCHARGE and to output a virtually DC voltage Vout.
This prior solution provides no control loop on the oscillator CLOCK of the multiplier, and its scan frequency is constant, independently of the current load I_Load on the regulator output. As this current load is varied, top efficiency can only be achieved at maximum load (I_Load_max), while at minimum (I_Load min) or zero load, unnecessary power consumption is brought about by the booster being kept fully operational. It is on this account that the above described solution is only applicable where the load I_Load is constant.
Another prior approach is schematically shown in FIG. 2. A fraction of the output voltage from the charge pump is used to drive a hysteresis comparator which turns on/off the oscillator CLOCK associated with the multiplier. Thus, according to the magnitude of the load I_load, the oscillator is set on or off to provide optimum efficiency.
Since no voltage regulator is provided in this approach, the output voltage Vout exhibits a ripplexe2x80x94at the same frequency as the oscillator CLOCKxe2x80x94which is added to an oscillation due to the range (drop_min; drop_max) defined by the comparator hysteresis.
The last-mentioned approach is more effective than the former, but a systematic ripple appears at the output Vout which is unacceptable in some applications, such as supplying an LCD driver. In this application the output voltage Vout must be programmed at different levels in steps of about 30mV.
An embodiment of this invention provides an electronic circuit for generating and regulating a supply voltage, which has such structural and functional features such to produce high boost efficiency and reasonable ripple in the voltage generated, thereby overcoming the drawbacks and limitations of the prior art.
The electronic circuit uses a pair of structurally and functionally independent hysteresis comparators which act on the oscillator of the voltage multiplier according to the values attained by the supply voltage produced out of the regulator, or attained by the voltage produced by the multiplier output.
According to one embodiment of the invention, an electronic circuit for generating and regulating a supply voltage is provided. The circuit includes a voltage multiplier that is associated with an oscillator, a voltage regulator having an input connected to the output of the voltage multiplier, a first hysteresis comparator having as inputs the regulator output and the multiplier output, and a second hysteresis comparator having as inputs a reference potential and a partition of the voltage presented on the regulator output, the outputs of both comparators being coupled to said oscillator.
The first comparator controls the circuit during normal operation. This comparator is configured to turn on the oscillator if the output of the multiplier does not exceed the output of the regulator by more a first margin, and to turn off the oscillator if the output of the multiplier exceeds the output of the regulator by more than a second margin.
The second oscillator controls the circuit during start-up, and holds the oscillator on until the output of the regulator reaches a programmed operating level. during this period the first comparator is disabled.
The invention also relates to a method of operation of the circuit, according to an embodiment of the invention..